It is either impossible or nearly impossible, as a practical matter, to build a rotating structure that is perfectly balanced upon manufacture. Any such structure will produce a certain amount of undesired vibration to a greater or lesser extent. Such vibration is usually passed through mounts that restrain the rotating part of the structure, and can therefore manifest itself as unwanted noise or vibration in adjacent structures.
A common example of this kind of problem is found in the modern high-bypass gas turbine engine presently used in commercial aviation. Vibration caused by unbalances in the various stages of such engine not only creates higher wear and fatigue in engine components and surrounding structures, but also causes unwanted noise in the passenger cabin of the airplane. Consequently, the manufacturers of such engines have developed special weights that can be affixed to the rotating fan and/or low pressure turbine (LPT) portions of each engine, as a means of balancing it, for controlling the magnitude of its unwanted vibration.
A person skilled in the art would know that the above-identified engine has numerous stages along its length. Typically, only the fan and LPT stages are accessible for applying weights after the engine is manufactured or assembled. Internal stages are inaccessible as a practical matter. Therefore, the specially developed balancing weights mentioned above are usable only for the fan and LPT stages. Some manufacturers provide corrective weights for the fan stage only, while others provide weights for both the fan and LPT stages.
Although fan and LPT unbalances, alone and by themselves, contribute to engine vibration as a whole, the unbalances that often reside at internal, inaccessible engine stages also contribute to overall engine vibration. When corrective weights can only be placed on the two accessible stages (fan and LPT), it is difficult to select weights of the proper magnitude and angular position such that they not only function to reduce vibration caused by the specific unbalances there, but also reduce the influence of unbalances at internal stages as well. Consequently, past methods of engine balancing have been frustrating, time consuming, and subject to a good deal of trial and error.
The balancing method disclosed here takes advantage of the modern digital computer, and an algorithm for solving what is mathematically known as a "minimization problem." While a "best" solution to engine vibration may not exist that can completely compensate for all sources or stages of engine unbalance, the present invention provides a "best" solution that will guarantee a lowest peak vibration for a range of engine RPMs.
The method disclosed here removes all trial and error guesswork to engine balancing. Although it was developed specifically for gas turbine engines, it is to be appreciated that such method could be applied to balance other types of rotating structures as well.